Transferrin receptor 1 (Tfr1) is a widely expressed protein that mediates cellular iron uptake through receptor-mediated endocytosis of Fe-transferrin. Tfr1-/- embryos die in mid-gestation from ineffective erythropoiesis, precluding assessment of roles of Tfr1 later in development and after birth. To determine whether Tfr1 is important in other tissues, we developed a conditional (floxed) Tfr1 mouse model that allows for inactivation of the Tfr1 gene in cells expressing transgenic Cre recombinase from tissue- specific promoters. We focused on skeletal and cardiac muscle because both tissues are highly metabolic and require iron for myoglobin production, mitochondrial biogenesis and mitochondrial function. Our preliminary results indicate that Tfr1 is critically important in both skeletal muscle and heart, and suggest an unanticipated link between iron homeostasis and energy metabolism. In our first Aim, we will characterize a severe phenotype observed in mice lacking Tfr1 in skeletal muscle, to understand how Tfr1 and iron transport contribute to muscle development, muscle iron homeostasis, mitochondrial biogenesis and regulation of energy metabolism. In our second Aim, we will perform similar studies to elucidate the role of Tfr1 in the heart, an organ in which iron balance must be tightly regulated to maintain constant, high-level function while avoiding deleterious effects of iron deficiency and iron overload.